This invention relates to containers made of thermoplastic material especially adapted to be filled with a hot liquid or semi-liquid product and hermetic sealed. The invention particularly relates to improvements in container design and filling operations so as to achieve a filled container which when cooled retains a desired container configuration despite the development of a partial vacuum within the container.
When a container is filled with a liquid or semi-liquid material, it is conventional packaging practice to leave a small, unfilled volume at the top of the container, called the head space. Typically, the head space of a fully filled container comprises about five percent of the total container volume. When such container filling occurs with the filling material in a hot condition, the air or gas in the head space is heated to about the same temperature as the filling material. After the container is filled, the container is quickly hermetically sealed with a cap preventing the influx of any air or gas within the container. Typically, such capping occurs within a matter of seconds so that no appreciable cooling of the container contents occurs prior to the hermitic sealing.
As the product within the container cools, the material forming the liquid or semi-liquid contents shrinks in volume thereby enlarging the head space typically by 80% or more. This increase in gas volume causes a decrease in pressure in general accord with Boyle's law. The drop in temperature of the gas within the head space also contributes to a decrease in pressure in general accordance with Gay-Lussac's law. The combined decrease in pressure is sufficient for a significant partial vacuum to develop. Thus, in containers having substantially fixed dimensions, such as those constructed of glass, a partial vacuum develops which may, be exhibited in an elastic inward deflection of the cap closing the fixed dimensional container. In containers having more flexible walls, the wall of the container often elastically deforms and at least partially collapses to the extent necessary to substantially reduce or eliminate the partial vacuum within the container. Such container deformation is commonly referred to as "paneling."
The paneling of a sealed container due to a cooling of the product within the container often makes the container appear misshapened, but generally does not detrimentally affect the quality of the product held by the container. However, there is often some consumer resistance to purchasing misshapened containers possibly based on a misguided assumption that the container is somehow less than full or the contents is no longer satisfactory. In addition to any consumer resistance to a randomly paneled container due to the undesirable appearance, it has been recognized that the container itself loses side wall symmetry and, hence, column strength which often prevents stacking of the containers for display or storage. Due in large part to cost considerations, it has been found desirable to lower the thickness of the side wall in the containers thus inherently making the side wall even more susceptible to vacuum induced paneling.
To diminish this consumer resistance to paneled containers, and to provide enhanced side wall symmetry so as to permit stacking, some containers have been designed to incorporate special features called vacuum deflection panels intended to be displaced inwardly in response to product shrinkage and cooling. The early solutions to the problem concentrated on providing structure for the ends, particularly the bottoms, of containers which would flex upwardly in response to a lowering of pressure within the container. More recently, various designs of side structures have been attempted to achieve the same result. Examples of such containers are to be found in U.S. Pat. Nos. 4,381,061; 4,542,029; 4,805,788; 4,863,046; and, 4,877,141. Many of these patents are directed to blow-molded containers of a biaxially oriented polymer such as polyethylene terephthalate resin (hereinafter referred to merely as PET). The PET containers are flexible yet self-supporting, that is, they do not collapse under their own weight.
Typically, the vacuum deflection panels in blow-molded PET containers of the prior art have been initially formed so they are slightly concave with the action of the partial vacuum within the container causing the panel to become increasingly concave as well as inwardly displaced. Generally, the side structure has consisted of inwardly indented panels adapted to flex still further inwardly into the container to offset the decrease in volume due to the cooling of the liquid product as well as the cooling of the gas within the head space. Despite the presence of the vacuum deflection panels, an inward deflection of a container wall can occur at a location other than at the vacuum deflection panel. To reduce the occurrence of misplaced container wall deflection, it has been found necessary to include various wall strengthening features and panel separating features so as to provide the desirable panel movement in response to the existence of the partial vacuum.
To ensure that the thin-walled PET containers have sufficient rigidity to withstand any panel collapse and to withstand the initial hot filling, the containers are typically thermally treated prior to filling and either during or subsequent to the blowing operation so as to enhance the stability of the container. This thermal treatment has the effect of rigidifying the container when cool so that the deflection of the panels becomes increasingly difficult. To offset this reduced deflection of each panel, the number of panels is increased to accommodate the required volumetric change in the container.
The vacuum deflection panels have often been included in a cylindrical body portion of a container, often referred to as the label panel where, typically, a label is applied to the container. While the presence of the label tends to hide the vacuum deflection panels, the presence of a large number of such panels significantly diminishes the label support thus making the label appear wrinkled or otherwise distorted thereby detracting from the product appearance.